Pipe installations employ valves to regulate fluid flow within the piping system. Valves can be hand operated or motor operated (MOV) or both; some valves can be pneumatically operated. Hand operated valves are opened and closed (stroked) by turning a handwheel coupled to a valve stem, which raises or lowers a disc to regulate fluid flow. Valve stems can be rising/non-rotating or non-rising/rotating and operate to stroke the valve in response to handwheel movement. Motor operated valves are stroked by operation of a motor coupled to the valve stem which operates (rotates or translates) in response to motor rotation. In either configuration, the handwheel or motor should be rotatable in both clockwise and counterclockwise directions to open or close the valve.
When pipes transport material, the load on the valve disc is a function of valve geometry and line pressure. The load determines the amount of force required to open and close the valve, which in turn determines the amount of torque required to engage the valve stem. The larger the fluid pressure inside a valve, the greater the torque required to stroke the valve.
To facilitate valve operation, traditional valve actuators employ a gear assembly to step down the input torque required to drive the valve stem. Such torque reducing gear assemblies find frequent use in applications where only manual operation via a handwheel is available, such as naval vessels where operational reliability is essential. In manual operation, the input force needed to provide the required torque to stroke the valve is determined by handwheel diameter. Handwheel diameter, however, can be constrained by the surrounding structure and machinery of the installation. Thus, gear assemblies are configured to accommodate the needed torque at a particular valve load given a specific handwheel.
The number of handwheel turns required to open or close a valve is determined by the gear ratio of the gear assembly. The greater the gear ratio, the lower the input force needed to impart a given torque, and the higher the number of handwheel turns needed to move the valve disc a given distance. Regardless of whether the load on the valve disc is low or high, though, the same number of turns are required to stroke the valve at a given gear ratio. While a large number of turns may be an acceptable trade-off when a heavy input torque is demanded, such a large number of turns car be inconvenient when the torque requirement is low because it requires a significant amount of time to stroke the valve.
Accordingly, there is a need for a valve actuator that can provide a variable gear ratio in response to different torque demands time constraints.